Various systems have already been proposed for evaluating such risks. The best known are the TAWS systems (acronym of the expression: “Terrain Awareness and Warning System”) developed for aircraft with a view to forestalling the risks of collision with the ground. TAWS systems make a short-term trajectory forecast for the aircraft on the basis of the flight information (position, heading, orientation and amplitude of the speed vector) provided by the onboard equipment, situate it with respect to a map of the region over flown extracted from a terrain elevation database accessible from onboard and emit alarms destined for the crew of the aircraft whenever the short-term forecastable trajectory collides with the ground. Certain TAWS systems augment their alarms with rudimentary recommendations of the kind “Terrain Ahead, Pull up”. Others also give information on the level of risk of collision incurred due to the relief's, the obstacles and the forbidden over flight zones present in the zone of deployment of the aircraft in the form of a map split up into regions of various colors depending on the significance of the risk incurred.
It is beneficial to go still further in navigational aid and to signal to the crew of a craft the potential dangers represented by certain parts of the domain of deployment of the craft even when they are not placed directly in the direction of movement of the craft, doing so in order to advise the crew of the craft as regards its freedom of deployment.
In this vein, the Applicant has proposed, in a French patent application FR 2,842,594 filed on Jul. 19, 2002, a system for preventing the risks of ground collision for aircraft, likening the risks of collision with the ground, to the penetration of the relief of the zone over flown into protection envelopes tied to the aircraft, quantifying them as a function of the time remaining for engaging a lateral avoidance turn of determined radius and displaying them on a navigation screen of the aircraft in the form of a map of the terrain zone over flown split up into regions of various colors corresponding to a risk scale. The Applicant has also proposed, in a French patent application filed on Dec. 19, 2003, under No. 0315020, that a maneuverability map representing the terrain zone over flown be displayed in the cockpit of an aircraft, as three types of region: a first type corresponding to the regions to be circumvented because they are considered to be uncrossable in view of the situation and of the present performance of the aircraft, for example, by a TAWS system or else because they form the subject of an over flight prohibition regulation, a second type corresponding to zones with restricted freedom of lateral deployment, constituted by marginal fringes surrounding the regions to be circumvented, of width corresponding to a minimum width necessary for the aircraft to describe, flat and at safety height, a diversion aerodrome and a third type of region complementary to the other two types, corresponding to zones of free lateral deployment. This navigation map, formulated on the basis of a terrain elevation database, implements a propagation distance transform for the placement of the outside contours of the zones with restricted freedom of lateral deployment.
Such a maneuverability map facilitates the navigation of an aircraft by signaling, in addition to the zones to be circumvented, the fringes of their neighborhood where the aircraft has its freedom of lateral deployment limited.
The Applicant has also proposed, in a French patent application filed on 16 Aug. 2004, under No. 04 06654, a system for signaling narrow passages of the trajectory of a flight plan where the freedom of lateral deployment of the aircraft is limited laterally by relief's or zones forming the subject of an over flight prohibition regulation. This system detects the risks of collision with the ground according to the procedure of the TAWS systems, by associating the risks of collision with the ground with the penetration of the relief or of forbidden zones into protection envelopes tied to the aircraft, containing a sufficient space to allow the aircraft to perform flat turns that are as tight as is permitted, while taking account of the wind.